In a conventional dry kiln used for drying lumber, a system of fans inside the kiln maintains a more or less uniform flow of air through the lumber. The moving air supplies heat to the wood and removes the wood's moisture, which is vented to the atmosphere outside of the kiln. The wet bulb and dry bulb temperatures of the air passing through the lumber are controlled, and various drying schedules can be used depending upon the moisture content of the wood, the wood species, and the end use of the lumber. Typically, air is circulated through the kiln in one direction, and after a period of time the air is circulated through the kiln in the opposite direction, thus maintaining uniformity of drying of the lumber. To maintain a desired humidity in the kiln, moisture laden air is exhausted through roof vents and as humid air is exhausted, drier colder outside air is drawn into the kiln and is heated within the kiln to operating temperature.
A conventional dry kiln wastes energy in several ways. For instance, humid exhaust air often is exhausted after it passes through heating coils within the dryer. This adds heat energy to the exhaust air, and this energy is wasted. Also, the incoming make-up air must be heated to accomplish the desired drying and pickup of moisture, which requires considerable heat energy for bringing the make-up air to its working temperature. These conditions lead to a more expensive operation in view of the energy actually required for drying.
Devices for pre-heating the make-up air with exhaust air are known. For example, U.S. Pat. No. 4,662,083 discloses a ventilation system that includes a flow control housing mounted to the roof of a dry kiln. The flow control housing includes a heat exchanger that removes heat from exhaust air vented from the kiln and transfers the heat to external make-up air before the make-up air is introduced into the kiln. A drawback of the system disclosed in the '083 patent is that a separate ventilation system is required for each dry kiln in a drying facility. If the facility has a large number of dry kilns, retrofitting each dry kiln can be cost prohibitive.
Accordingly, there is a continuing need for new and improved ventilation systems for dry kilns.